Morphogens provide the positional information that is used to form patterns
of distinct cell types in developing tissues. But can morphogen gradients
alone determine these precise patterns or do they establish a relatively
coarse pattern that is refined later? A recent study of the Bicoid gradient in
fly embryos supports the first theory. However, for the Decapentaplegic (Dpp)
morphogen gradient in developing Drosophila wing discs, Bollenbach
and colleagues now suggest that the second scenario is more appropriate (see
p. 1137). These
researchers have developed a theoretical description of Dpp gradient formation
and have also measured Dpp gradients, Dpp signalling activity and the
activation of the Dpp target gene spalt (which controls vein pattern)
in developing wing discs. These approaches indicate that the Dpp gradient
positions the Spalt boundary to an accuracy of about three cells in the discs.
Because the final vein pattern is more precise than this, the researchers
suggest that downstream events must refine the positional information provided
by the Dpp gradient.

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